The invention relates to optical communication systems, and more specifically to methods and apparatus for utilizing optical transmission fibers which support higher order spatial modes.
Multimode fibers which typically support hundreds of optical modes are subject to modal dispersion. Single-mode optical fibers (SMF) which exclusively support a single optical spatial mode, namely the LP01 spatial mode also known as the fundamental mode, are typically used today in optical communication systems. The transmission loss in these SMFs is generally minimized for wavelengths in the range of 1300 and 1550 nm typically utilized for long-distance communications. Single mode fibers are typically utilized because they exhibit virtually no signal quality degradation as a result of modal dispersion. However, as the pulses in this range propagate through an SMF, their waveforms tend to spread because of chromatic dispersion resulting in limitations on the bit rate and the transmission distance. The total chromatic dispersion experienced is a combination of material dispersion and waveguide dispersion, which may be of opposite sign. In a conventional non-dispersion shifted SMF commonly used in conjunction with the communication wavelength band of 1550 nm, the total dispersion is approximately 17 ps/nmxc2x7km. This primarily results from material dispersion.
An additional limitation in the transmission of optical signals is the existence of noise, which masks the signal. The signal-to-noise ratio (S/N) is a critical factor in allowing either lower power or longer distances for transmission. Current fiber technology does not typically address this problem, and it is instead resolved by regenerating the signal as necessary with amplification or signal conditioning, which adds expense and complexity to the system.
Dispersion slope is defined as the rate of change of the total chromatic dispersion of the fiber as the wavelength changes. In a conventional non-shifted SMF used for the communication wavelength band of 1550 nm the dispersion slope is about 0.06 ps/nm2xc2x7km. Compensating for positive dispersion by using dispersion compensating fibers which have low attenuation and high negative dispersion values is well known in the art (See, for example, U.S. Pat. Nos. 5,185,827, 5,261,016 and 5,361,319). However, compensating for negative dispersion may require long lengths of fiber (See, for example, U.S. Pat. No. 4,261,639). Most systems utilize some method of compensating for dispersion, so that the received signal is not substantially dispersed or spread. However, the use of dedicated dispersion compensating fibers adds losses to the system. In addition, compensating for the dispersion slope is quite difficult, and as a result, the channels furthest from the center channel in wave division multiplexed (WDM) systems experience residual dispersion even after compensation.
A paper entitled. xe2x80x9cSubmarine Fiber Design for 10 GB/S Transmissionxe2x80x9d delivered at the 1999 Annual Multiplexed Telephony Conference by Atwood and Adcox of Corning Incorporated, Corning, N.Y., describes a current method being utilized to compensate for dispersion without utilizing a dedicated dispersion compensating fiber. In the system described in the paper, high dispersion is desired locally throughout the system and dispersion close to zero is desired for the overall system or repeating block. Nine spans of negative dispersion fiber are used, and are compensated by a single span of positive dispersion fiber. This method is well known to those skilled in the art and is referred to as concatenation. A span is defined as the length of fiber between amplifiers, or between the transmitter or receiver and the nearest amplifier. While the dispersion is compensated at some wavelengths, the slope is not correctly compensated for and must be dealt with on an individual wavelength basis after demultiplexing. Furthermore, a single mode fiber (SMF) with negative slope typically has a small effective area (Aeff), which leads to increased undesired non-linear effects in high power systems. To solve this problem, a complicated solution is employed. The solution requires the use of a subspan of a large effective area fiber, which has a large dispersion slope at the beginning of the span for the effective length of nonlinear interactions, followed by a subspan of a different fiber profile for the balance of the span.
U.S. Pat. No. 5,781,673 describes another solution for resolving the dispersion slope by utilizing an additional span of dispersion slope compensation fiber. However, this solution adds complexity to the system, and may enhance undesirable losses as well.
Thus there is a need for a system utilizing fibers with a large effective area (Aeff) for reduced non-linear effects, minimal dispersion and dispersion slope. It is also desirable to design a fiber that is flexible enough to exhibit both positive and negative dispersion, and both positive and negative slope. There is also a need for a method to reduce noise in an optical transmission system.
The invention relates, in one embodiment, to an optical transmission system for transmitting an optical signal having optical energy. The system, in another embodiment, includes at least one transmission span including an optical waveguide. The transmission span transmits substantially all of the optical energy in a single high order spatial mode. In another embodiment, the single high order spatial mode is the LP02 spatial mode. The optical transmission system, in yet another embodiment, further includes at least one additional transmission span. The transmission span, in another embodiment, includes an optical fiber.
The optical wave guide, in one embodiment, has a dispersion and a dispersion slope for a given transmission bandwidth. The dispersion includes at least material dispersion, and the absolute value of the dispersion is substantially between zero and the material dispersion. In another embodiment, the dispersion of the optical waveguide is negative over the transmission bandwidth. In yet another embodiment, the dispersion of the optical waveguide is positive over the transmission bandwidth. In still other embodiments, the dispersion slope of the optical waveguide is positive, negative, or nominally zero.
The invention further relates to an optical transmission system which includes a spatial mode transformer positioned to receive an optical signal. The spatial mode transformer transforms the optical energy of the optical signal from a low order spatial mode to a high order spatial mode. The system further includes an optical transmission waveguide in optical communication with a spatial mode transformer, and the optical transmission waveguide transmits substantially all of the optical energy in the high order spatial mode. In one embodiment, the high order spatial mode is the LP02 spatial mode. In another embodiment, the low order spatial mode is the fundamental spatial mode. In yet another embodiment, the system includes a second spatial mode transformer in optical communication with the optical waveguide, and the second spatial mode transformer transforms the optical energy in the high order spatial mode to a low order spatial mode.
The invention further relates to a method for transmitting an optical signal having optical energy substantially in a single high order spatial mode. The method includes the steps of receiving the optical signal having optical energy in the single high order spatial mode, and transmitting the optical signal having optical energy in the single high order spatial mode through a transmission span. The transmission span includes an optical waveguide.
Another aspect of the invention includes a method for transmitting an optical signal having optical energy in a first spatial mode. The method includes the steps of receiving an optical signal having optical energy in the first spatial mode, transforming the optical energy in the first spatial mode to optical energy in a second spatial mode, and transmitting the optical energy in the second spatial mode through an optical transmission waveguide. In another embodiment, the method further includes the step of transforming the optical energy in the second spatial mode to optical energy in a third spatial mode. In one embodiment, the first spatial mode is the LP01 spatial mode. In another embodiment, the second spatial mode is the LP02 spatial mode. In yet another embodiment, the third spatial mode is the LP01 spatial mode.